Control system for the pre dryer section of a paper machine

ABSTRACT

SYSTEM FOR CONTROLLING THE PRESSURE OF STEAM INTRODUCED INTO A PAPER DRYING DRUM IN SUCH A WAY THAT THE TEMPERATURE OF THE PAPER WEB LEAVING THE DRUM WILL NE CONSTANT. THIS IS ACCOMPLISHED BY DERIVING A PREDICTIVE FEED-FORWARD SIGNAL FROM CONSIDERATION OF A HEAT BALANCE AROUND THE DRUM AND BY VARYING THE PREDICTIVE SIGNAL WITH A FEEDBACK SIGNAL PROPORTIONAL TO ACTUAL EXIT TEMPERATURE. THE MODIFIED FEED-FORWARD SIGNAL IS THEN USED AS AN ERROR SIGNAL FOR CONTROLLING THE STEAM PRESSURE WITHIN THE DRUM, AND, HENCE, ITS TEMPERATURE.   D R A W I N G

July 24, 1973 W. ROBERTUS J. TlLLlE ETAL CONTROL SYSTEM FOR THEPRE-DRYER SECTION OF A PAPER MACHINE Filed Nov. 24, 1970 T35 T2 T =T T P2 F I ,3

1 4o 34 l I I1 E 46 x U 1 T3 T 3m, ,3

4' X P 5%;? I TABLE 60 50 59 I XKI 32 t 54 28 PRESSURE J CONTROLLERSTEAM ,5 H /24 IN y so CONDENSATE MEASUREMENT.

62 u n 2 3 Q x uacouo. i l l l l NVA I Tp T2 A I x Tp- T3 U;

United States Patent 3,748,224 CONTROL SYSTEM FOR THE PRE-DRYER SECTIONOF A PAPER MACHINE W. Robertus J. Tillie, New York, N.Y., and James T.

Carleton, Pittsburgh, and Richard E. J. Putman, Penn Hills, Pa.,assignors to Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed Nov. 24, 1970, Ser. No. 92,347

Int. Cl. D21f 5/06; G06f 15/46 U.S. Cl. 162-252 5 Claims ABSTRACT OF THEDISCLOSURE System for controlling the pressure of steam introduced intoa paper drying drum in such a way that the temperature of the paper webleaving the drum will be constant. This is accomplished by deriving apredictive feed-forward signal from consideration of a heat balancearound the drum and by varying the predictive signal with a feedbacksignal proportional to actual exit temperature. The modifiedfeed-forward signal is then used as an error signal for controlling thesteam pressure within the drum, and, hence, its temperature.

BACKGROUND OF THE INVENTION In the manufacture of paper from wood pulp,the pulp in a head box passes through a slice gap or slit onto a wirecomprising a fine wire screen in the form of an endless belt, usuallyseveral feet wide. Vibration of the screen spreads the material in athin uniform layer or web. As it is carried along on the wire, much ofthe water in the soft mass drains out. To prevent the material fromrunning over the edges of the wire, rubber belts are usually placed oneither side. Mechanical suction removes much of the water from the wetpulp mass, after which the web passes between rollers to a thick woolenfelt which is carried between a series of heaw rollers. Passing betweenthese, the paper is pressed into a firm thin sheet, and thereafterpasses over a series of heated drying rolls. Usually, the Web passesover the initial rolls while it is still on the felt mat; however, aftersuflicient drying has taken place, the web may be separated from thefelt mat and passed over the remaining drying rolls to a sizing presswithout any external support.

The initial drying roll or rolls over which the web passes, areordinarily referred to as the pre-dryer. The main function of thepre-dryer is to heat the pulp web or sheet to a predeterminedtemperature. It has been found from actual operating experience that ifthe web, upon leaving the pre-dryer, is at a certain desired temperature(which in one case is 160 F.), then the remaining dryers can be operatedmuch more efiiciently. There is no theoretical justification for this;it is something that must be found out from experience.

Thus, it is of primary importance to hold the temperature of the sheet,as it emerges from the pre-dryer, at a constant, predeterminedtemperature. Introducing a set amount of steam into the pre-dryer drumor drums Will not give a constant output temperature for the reason thatthe moisture content of the sheet, its density, its velocity, the amountof air blowing over the drum, and other factors will vary, meaning thatthe amount of heat and, hence, the amount of steam needed to maintain afixed output temperature will vary. Furthermore, a system which attemptsto control output temperature by measuring the temperature of theissuing web is highly unsatisfactory since, among other thin-gs, itconstitutes control after the fact rather than before the fact. That is,if an attempt is made to control the steam input to the drying drum as afunction of exit temperature only, the

Patented July 24, 1973 "Ice measuring instrument, which is spaced fromthe roll, will measure the temperature after the strip has traveled fromthe drum; but during this time the conditions of pulp consistency,velocity and the like may have changed entirely.

SUMMARY OF THE INVENTION In accordance with the present invention, thetemperature of pulp issuing from the pre-drying drum of a papermakingmachine is controlled by conducting a heat balance around the drum togenerate a feed-forward signal indicative of the pressure andtemperature of the steam necessary to raise the temperature of the webfrom a known input temperature to a desired output temperature. Thisfeed-forward signal is then modified by a feedback signal derived bycomparison of desired output temperature with the actual outputtemperature of the web to derive an error signal for controlling thepressure of the steam in the drying drum.

The feed-forward signal is generated from a consideration of the factthat the heat, Q required to heat up the paper web from an incomingtemperature, T to an outgoing temperature, T equals the heat, Qtransferred from the inside of the dryer drum or drums to the web. Theheat, Q transferred from the inside of the drum to the Web can becalculated from a consideration of the heat transfer characteristics ofthe drum, the temperature of the steam within the drum, and the sheettemperatures, T and T at the input and output of the drying drum ordrums, respectively. Q the heat required to heat up the paper web fromtemperature T to temperature T can be calculated from the mass flowrates of pulp and water flowing onto the drying drum, the specific heatsof the fiber and water, and the temperatures T and T Since Q must alwaysbe equal to Q an equation can be derived for determining thetemperature, T,,, of the steam. This temperature then comprises afeed-forward signal which is modified by a comparison of the desiredoutput temperature with actual output temperature to vary the steam flowinto the pre-dryer drum.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying single figure drawing which schematicallyillustrates one embodiment of the invention.

With reference now to the drawing, the embodiment of the invention shownincludes a felt belt 10 which carries, on its upper surface, a moist Webof pulp 12 which is to be dried and pressed into paper sheet. The felt,with the moist pulp thereon, passes over a pre-dryer 14 comprising arotatable drum having a hollow interior into which steam from line 16 isintroduced. The exhausted steam, in turn, passes out of the hollowinterior of the drum 14 through conduit 18 to a condenser or separator20.

As was mentioned above, the pre-dryer drum 14 is the first drying drumin the process of drying and rolling pulp into paper and may comprise aseries of drums rather than the single drum shown herein. The drumnormally comprises a cast iron cylinder with a wall thickness of about1% inches, the heat of the steam within the hollow interior of the drumbeing transferred through the wall of the drum to the felt 10 and pulp12. In order to obtain maximum efiiciency in subsequent dryingoperations, it is desired that the pulp issuing from the pre-dryer 14have a predetermined temperature which, in one specific example, is F.

The temperature of the pulp issuing from the predryer is measured by atemperature measuring device 22 which produces an electrical signal onlead 24 proportional to that temperature. Similarly, the temperature ofthe incoming pulp is measured by a temperature measuring device 26 whichproduces anelectrical signal on lead 28 proportional to the measuredtemperature. In addition, the density of the pulp on felt is measuredand converted into a proportional electrical signal by a beta gauge 30.This electrical signal is multiplied by a constant, K at 32 to produce asignal M which is proportional to the pounds of dry fiber pulp perminute passing along the felt belt 10. This, of course, assumes aconstant speed of the belt 10. If the speed of the belt should vary,then a tachometer generator and compensating circuitry will have to beemployed for variations in speed.

As was mentioned above, it is desired to maintain the outputtemperature, T at a desired specific value. Furthermore, the heat, Qrequired to heat up the pulp from the incoming temperature T; to theoutgoing temperature T equals the heat, Q transferred from the inside ofthe dryer drum to the web. Mathematically, this can be expressed asfollows:

N=nurnber of rolls in the pre-dryer (one in the example given);

U=overall heat transfer coefiicient (B.t.u./ F. -ft. -min.);

A=contact area of the sheet and roll (ft?) T =calculated desiredtemperature of steam in drum T =actual input temperature of pulp F.);and

T =desired output temperature of pulp F.).

Q can be calculated from: Q2=( w w+ nb nb)X( a* 2) where M =moisture inincoming sheet (lbs./min.); C =specific heat of water (B.t.u./lb.- F.);M =bone dry pulp rate (lbs/min); and C =specific heat of dry pulp(B.t.u./lb.- F.).

Thus, the desired steam temperature T,,* can be calculated from Equation14; and this is performed in computer circuitry identified by thereference numeral 34 in the drawing to produce an electrical quantitycomprising the feed-forward signal. The computer circuitry 34, as wellas the other circuit blocks shown herein, may comprise part of a generalpurpose computer which is programmed to perform the calculation ofEquation 14 and the other control functions described herein to producean electrical quantity comprising the feed-forward signal. However, itis shown as hard-wired logic in the drawing to facilitate explanation.

The quantity M is derived in block 36 by multiplying M times a constantK. That is, it is assumed that a certain rate of dry pulp flowing to thedrum 14 will contain a fixed percentage of water. The quantity T isentered into the computer circuitry 34 from block 38 and is selected byan operator.

Since the parameter to be controlled is the actual exit temperature Tthis temperature is compared with the desired temperature T at 40 andthe quotient or ratio subtracted from 1 in block 42 to derive acorrective feedback signal which is integrated in integrating controller44 and multiplied with the quantity T,,* at the output of circuit 34 at46. This corrected value T is then applied to circuit 48 where it ismultiplied by a factor 1 from a steam table 50 stored in the computermemory to derive P*. P* is the desired pressure of the steam within thedrum 14 to achieve an exit temperature equal to T The actual steam fedinto the drum 14 is controlled by means of valve 52 connected to apressure controller 54 which receives a set point signal fromintegrating controller 56. The set point of controller 56, in turn, isderived by comparing in comparator 60 P* from block 60 with the actualsteam pressure P as derived from a pressure sensing device 58. Thus, aservo loop is established in which any change in the value of P* willcause an adjustment in the setting of valve 52 which, in turn, will varythe output of pressure sensing device 58 until the two signals, P* andP, are equal, at which time the pressure in the drum 14 is equal to thecalculated desired pressure P*.

In the foregoing Equation 2, the quantity U, the overall heat transfercoeflicient, includes losses to the air blowing across the pulp andperhaps the tightness of the felt and other factors. For this reason,the quantity U is not a fixed value but changes and, consequently, it isdesirable to update the quantity U. This is achieved by measuring theamount of condensate coming out of the dryer at 60 and converting thisat 62 into the amount of heat actually given up by the steam bymultiplying pounds of condensate by a constant K This, then, can becompared with calculated heat transferred, Q in circuitry 64 and thedifference derived at 66 integrated at 68 and used to multiply U*, thepredicted heat transfer coefficient, at 70. This gives a corrected valueof U which is fed back to circuit 34.

Although the invention has been shown in connection with a certainspecific embodiment, it will be readily apparent to those skilled in theart that various changes in form and arrangement of parts may be made tosuit requirements without departing from the spirit and scope of theinvention.

We claim as our invention:

1. In papermaking apparatus of the type wherein a web of moist pulp isheated by passing it over a hollow drying drum charged with steamthrough a steam supply line having a control valve therein whereby theheat of the steam is transferred through the wall of the drum to thepulp, the improvement comprising a system for maintaining thetemperature of the pulp issuing from the drum at a preset value, saidsystem including:

means for computing the desired temperature of the steam Within saiddrum from a consideration of the heat required to heat up the pulp froma known input temperature to a desired output temperature and the heattransferred from the interior of the drum to the pulp, said means forcomputing the desired temperature of the steam within said drumincluding a device for measuring the actual temperature of pulp passingtothe drying drum and means for determining the weight per unit of timeof dry pulp and moisture passing to the drying drum, means responsive tothe desired steam temperature thus computed for generating afeed-forward error signal for controlling the admission of steam intosaid drum,

means for comparing the actual output temperature of said pulp with saiddesired output temperature to derive a feedback error signal whichmodifies said feed-forward signal until the desired and actualtemperatures are the same, and

valve control means responsive to said feed-forward signal as and ifmodified for controlling said control valve and, hence, the admission ofsteam into said drying drum.

2. The improvement of claim 1 wherein said computed desired temperatureof the steam within said drum is compared with temperatures stored in asteam table to derive a feed-forward error signal proportional to thedesired pressure of the steam within said drum.

3. The improvement of claim 1 wherein the means for computing thedesired temperature of the steam within said drum computes saidtemperature from the equation:

where T,,* is the desired temperature of the steam within said drum, Tis the temperature of the pulp passing to the drying drum; T is thedesired output temperature of the pulp as determined by an operator; Nis the number of rolls over which the pulp passes, U is the overall heattransfer coefiicient of the drying drum, A is the contact area of thepulp with the periphery of the drying drum, M is the bone dry pulp rate,C is the specific heat of the dry pulp, M is the moisture in theincoming sheet, and C is the specific heat of water and F is N -U-A w w+fib+ ib 4. The improvement of claim 3 wherein said means for computingthe desired temperature T,,* produces an electrical signal proportionalto T,,* and includes means for comparing an electrical signalproportional to T with an electrical signal T proportional to actualoutput temperature to derive an error signal for modifying theelectrical signal proportional to T,,*.

5. The improvement of claim 4 wherein said electrical signalproportional to T,,* is compared with known information stored in asteam table for deriving an electrical signal proportional to P which isthe desired pressure of the steam within said drying drum.

References Cited UNITED STATES PATENTS 3,564,724 2/ 1971 Keyes et al34-48 3,518,775 7/1970 Bartles et a1 162-198 X 3,295,842 1/ 1967Stelling et a1 34-48 X 3,216,241 11/1965 Hansen 73-75 3,363,325 1/1968Schie 34-41 2,309,993 2/ 1943 Skagerberg 34-41 X 2,922,475 1/1960Alexander 162-252 S. LEON BASHORE, Primary Examiner R. H. TUSHIN,Assistant Examiner US. Cl. X.R.

